In order to use the C code in these directories, you should first
modify / setup the file config/LocalMakefileConfig.mk, which is
included by the main Makefiles and allows you to overwrite the
settings from config/Makefile.mk.

Once this is done, run make.

You must then use the install targets to copy / link the resulting
shared libraries with their full paths into the "luatex/lib"
subdirectory of the directory where your TeX executable resides. For
instance, if the luatex program is in 

/usr/texbin

then you would put SimpleDemoC.so into

/usr/texbin/lib/luatex/lua/pgf/gd/examples/c


Due to bugs in the LuaTeX code, you currently need to append the paths
of the libraries to their name as done in the Makefiles.

Note that, currently, the Makefile will link the Lua library
statically into the shared graph drawing library (at least on
MacOS). This is conceptually wrong and, indeed, a lot of effort was
need to avoid having LuaTeX crash on a TeX run because of two Lua
libraries being used simultaneously. It works, but hopefully, in the
fututure, this will be fixed.